1. Field of the Invention
The present invention relates in general to video recording/playback systems, and an apparatus and a method for automatically controlling pulse generation, in which an error of a head switching signal resulting from a positioning error of a pulse generating head in a drum is accurately corrected.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a conventional pulse generation control apparatus in a block form. As shown in this drawing, the conventional pulse generation control apparatus comprises a drum 1 with a pair of video heads H1 and H2, a pulse generating head 1a and a frequency generating head 1b. The video heads H1 and H2 are adapted to play back video signals recorded on a magnetic tape. The pulse and frequency generating heads 1a and 1b are adapted to generate a pulse generation signal PG and a frequency generation signal FG, respectively, to accurately play back the recorded video signals.
The conventional pulse generation control apparatus also comprises wave-shapers 2 and 3 for wave-shaping the pulse generation signal PG from the pulse generating head 1a and the frequency generation signal FG from the frequency generating head 1b, respectively, and a mono-multi signal generating circuit 4 for generating a mono-multi signal MM with its rising curve time t varied based on a time constant of a variable resistor VR and a condenser C.
A servo device 5 is provided to input the wave-shaped pulse generation signal and frequency generation signal from the wave-shapers 2 and 3 and the mono-multi signal MM from the mono-multi signal generating circuit 4, to output a head switching signal H S/W in response to the inputted mono-multi signal and to output through an amplifier 6 a drum control signal in response to the inputted pulse generation signal and frequency generation signal and the head switching signal H S/W, to control the speed and phase of the drum 1.
The conventional pulse generation control apparatus also comprises a pre-amplifying circuit 7 for amplifying the video signals played back by the video heads H1 and H2, and a luminance/chrominance processing circuit 8 for selecting a channel for inputting one of the video signals from the pre-amplifying circuit 7 in response to the head switching signal H S/W from the servo device 5, processing luminance and chrominance of the video signal input from the selected channel and outputting the processed video signal.
The operation of the conventional pulse generation control apparatus will hereinafter be described with reference to FIGS. 2A to 2E.
As the drum 1 is rotated, the video heads H1 and H2 play back the video signals recorded on the magnetic tape and the pulse generating head 1a and the frequency generating head 1b generate the pulse generation signal PG and the frequency generation signal FG, respectively. Namely, the pulse generation signal PG is outputted from the pulse generating head 1a, as shown in FIG. 2A, and then wave-shaped as shown in FIG. 2B by the wave-shaper 2. Also, the frequency generation signal FG is outputted from the frequency generating head 1b and then wave-shaped by the wave-shaper 3.
The wave-shaped pulse generation signal PG and frequency generation signal FG from the wave-shapers 2 and 3 are applied to the servo device 5, which also inputs the mono-multi signal MM from the mono-multi signal generating circuit 4, the rising curve time t of which is varied based on the time constant of the variable resistor VR and the condenser C.
A voltage level V of the mono-multi signal MM as shown in FIG. 2C can be expressed by the following equation: EQU V=1-e.sup.-(1/RC)t
The rising curve time t of the mono-multi signal MM is varied according to a variation of the variable resistor VR.
The head switching signal H S/W is outputted from the servo device 5, as shown in FIG. 2D, to control the rotational phase of the drum 1. This head switching signal H S/W from the servo device 5 becomes high at a falling edge of the mono-multi signal MM from the mono-multi signal generating circuit 4 and goes low at 30 Hz in the case of a NTSC system and 25 Hz in the case of a PAL system.
The video signals played back by the video heads H1 and H2 are amplified by the pre-amplifying circuit 7 and then applied to the luminance/chrominance processing circuit 8. The luminance/chrominance processing circuit 8 serves to select a channel for inputting one of the video signals from the pre-amplifying circuit 7 in response to the head switching signal H S/W from the servo device 5. Also, as shown in FIG. 2E, the luminance/chrominance processing circuit 8 processes the luminance and chrominance of the video signal input from the selected channel and outputs the processed video signal field scan signals (whose envelopes are indicated by rectangles containing an "X") separated by vertical blanking intervals, each of which contain the vertical synchronization signal, Vs.
The control of the phase of the drum 1 is performed by adjusting an interval between a rising edge of the head switching signal H S/W as shown in FIG. 2D and the next subsequent edge of the vertical synchronous signal Vs of the video signal to 6.5 H (where H is the time interval between horizontal synchronization pulses in a television signal) as shown in FIG. 2E. The interval between the rising edge of the head switching signal H S/W and the next subsequent edge of the vertical synchronous signal Vs of the video signal is determined by adjusting the rising curve time t of the mono-multi signal MM, which is determined by adjusting the variable resistor VR in the mono-multi signal generating circuit 4.
Errors in the rotational speed and phase of the drum 1 are corrected by the adjustment of the head switching signal H S/W. The servo device 5 outputs the drum control signal to correct the errors. The drum control signal from the servo device 5 is amplified by the amplifier 6 and then applied to the drum 1, thereby causing the speed and phase of the drum 1 to be controlled.
However, the above-mentioned conventional pulse generation control apparatus has a disadvantage in that it is impossible to accurately correct the head switching signal since an error of the head switching signal resulting from a positioning error of the pulse generating head in the drum is corrected by adjusting the variable resistor. This results in an inaccurate correction of the error in the phase of the drum.